A method for forming a semiconductor device structure is provided. The method includes depositing a gate dielectric layer over a substrate. The substrate has a base portion and a first fin portion over the base portion, and the gate dielectric layer is over the first fin portion. The method includes forming a gate electrode layer over the gate dielectric layer. The gate electrode layer includes fluorine. The method includes annealing the gate electrode layer and the gate dielectric layer so that fluorine from the gate electrode layer diffuses into the gate dielectric layer.